Scruffy nerf herder
Toaster Loving AdMech Boi
My only rule as DM is that what happens in character stays in character and doesn't spill over to the table.
Follow that rule, along with common decency, and pretty much anything goes. Experience tells me that every so often in a long campaign some big in-character argument or fight will rear its head, then once it's out of their systems things settle down again.
I neither expect nor demand on any sort of meta-level that the PCs in my game aspire to any sort of hero status - they go on the adventures they go on and do what they do, and while some heroism might tangentially result from that (and some characters might bask in it!) if the party instead decides to throw in with the bad guys or whatever it's all fine with me.
IME it's the first few adventures in a long campaign when players aren't as invested in their characters that can really be a powderkeg, but with the right players the resulting shenanigans can generate some amazing fun and hilarity.
Mmmmm-mmmm-mmmm, this post goes down like fried chicken! Look I'm not interested in putting down folks who can't understand the appeal of playing like this, but you're explaining such a bog standard kind of campaign (at least in terms of my own experiences), and yet it's so "subversive" by many people's standards it's like you're reinventing the wheel. We, i.e. people who have fun playing in all evil campaigns, are playing Dungeons N Hamsters the same as all you guys.
You don't have to be an alien or something to think it's fun being a Sith when you play Knights of the Old Republic. It's a game. The heroic/normal campaign is still chocked full of stuff that is bewilderingly different from how a person would act irl. Morality is already a wacky mess if you just try lining up your ideas about what's really good and just, in the real world, with the things that "good guys" do in campaigns.
Wouldn't a lot of us be pacifists if in the real world there was magic and stuff? We wouldn't honestly go out adventuring and killing "for good", would we? We'd think those "good" adventurers are insane and wish that our "actually good" town guards would throw them out before they cause bloodshed in town.